Graded index type optical fibers and method of making the same

ABSTRACT

A graded index type optical fiber having a multilayer structure comprising a plurality of concentrically arranged layers formed of (co)polymers selected from the group consisting of two or more homopolymers HP 1,  HP 2,  . . . , HPn derived from monomers M 1,  M 2,  . . . , Mn, respectively, and having refractive indices decreasing in that order, and one or more binary copolymers CP derived from these monomers, the multilayer structure being such that a mixed layer consisting of the (co)polymers constituting two adjacent layers is formed therebetween, and the refractive index is highest at the center and decreases gradually toward the outer periphery. This optical fiber can be continuously formed by feeding the (co)polymers to a multilayer concentric circular nozzle and thereby extruding them through the nozzle, and allowing the polymers to interdiffuse between adjacent layers of the fiber.

This application is a continuation of application Ser. No. 09/142,161,filed Sep. 28, 1998, now U.S. Pat. No. 6,185,353, issued on Feb. 6, 2001which is a 371 of PCT/JP97/01093, filed Mar. 28, 1997.

TECHNICAL FIELD

This invention relates to graded index type plastic optical fibers whichcan be used as optical communication media.

BACKGROUND ART

Graded index type plastic optical fibers (hereinafter referred to as “GItype POFs”) having a radial refractive index distribution in which therefractive index decreases gradually from the center toward the outerperiphery of the optical fiber have a wider frequency bandwidth thanstep index type optical fibers, and are hence expected to be useful asoptical communication media.

In the case of GI type POFs, one having a large numerical aperture (NA)and as small a transmission loss as possible needs to be manufacturedfor the purpose of improving its bending loss and its coupling loss withthe light source. In order to increase NA, GI type POFs must be designedso that the maximum difference in refractive index (Δn) between thecenter and the outer periphery of the optical fiber is sufficientlylarge.

Various methods of making such GI type POFs are known. They include, forexample, (1) a method which comprises providing two monomers havingdifferent reactivity ratios and giving homopolymers with differentrefractive indices, placing these monomers in a cylindrical vessel madeof a polymer of these monomers so as to cause the polymer to bedissolved and swollen, polymerizing the monomers, and then drawing theresulting product (Japanese Patent Laid-Open No. 130904/'86); (2) amethod which comprises preparing a plurality of polymer mixtures fromtwo polymers having different refractive indices at various mixingratios, spinning these polymer mixtures to form a multilayer fiber, andthen heat-treating this fiber to effect interdiffusion between adjacentlayers (Japanese Patent Laid-Open No. 265208/'89); and (3) a methodwhich comprises winding films formed of a plurality of binary copolymershaving different copolymerization ratios on a core material, and drawingthe resulting laminate under heated conditions (Japanese PatentPublication No. 15684/'80).

The GI type POFs made by the above-described methods (1) or (2) have thedisadvantage that, since all layers are formed of polymer mixtures,these plastic optical fibers (hereinafter referred to as “POFs”) tend toproduce a heterogeneous structure due to microscopic phase separationand hence show a large light scattering loss. On the other hand, the GItype POFs made by the method (3) and consisting of styrene-methylmethacrylate copolymers or the like have a large light scattering loss,because the difference in refractive index between the copolymersconstituting adjacent layers of the multilayer fiber is too large (e.g.,0.02).

As the methods of making, the above-described method (1) isdisadvantageous in that it requires a polymerization step and hence haslow productivity. The method (3) is disadvantageous in that foreignmatter tends to be introduced when a plurality of films are wound on acore material and in that it is difficult to obtain a concentriccircular fiber because thickness discontinuities tend to occur at thejoints between film ends.

On the other hand, the method (2) is excellent in that a GI type POFshowing few thickness fluctuation can be continuously formed. However,it is difficult to create a gradual refractive index distribution in thePOF, because sufficient polymer-to-polymer interdiffusion betweenadjacent layers cannot be achieved by the post-spinning heat treatmentalone. Even if the heat-treating temperature is raised to increase thethickness of the interdiffusion layers and thereby to create a gradualrefractive index distribution profile, the fiber drawn during spinningtends to undergo relaxation shrinkage and show variations in fiberdiameter. Consequently, light leakage and scattering occur in the partsshowing variation in diameter, resulting in an increased transmissionloss.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a technique by which GItype POFs showing few thickness fluctuation and having a small lightscattering loss and a relatively large numerical aperture can be made ata high production rate.

According to the present invention, there is provided a graded indextype optical fiber having a multilayer structure comprising a pluralityof concentrically arranged layers each of said layers is formed of one(co)polymer selected from the group consisting of two or morehomopolymers HP1, HP2, . . . , HPn (in which n is an integer of 2 orgreater) derived from monomers M1, M2, . . . , Mn, respectively, andhaving refractive indices decreasing in that order, and one or morebinary copolymers CPs derived from the monomers, the multilayerstructure being such that a mixed layer consisting of mixture of two(co)polymers constituting two adjacent layers is formed therebetween,and the refractive index is highest at the center and decreasesgradually toward the outer periphery.

According to the present invention, there is also provided a method ofmaking a graded index type optical fiber which comprises the steps ofpreparing a plurality of spinning materials having different refractiveindices, each of said spinning materials being made of one (co)polymer,by using (co)polymers selected from the group consisting of two or morehomopolymers HP1, HP2, . . . , HPn (in which n is an integer of 2 orgreater) derived from monomers M1, M2, . . . , Mn, respectively, andhaving refractive indices decreasing in that order, and one or morebinary copolymers CPs derived from the monomers; feeding the spinningmaterials to a multilayer concentric circular nozzle so that therefractive index decreases toward the outer periphery, and therebyextruding them through the nozzle; and allowing the polymers tointerdiffuse between adjacent layers of the fiber, within the nozzleand/or after being extruded from the nozzle.

In the aforesaid POF and its method of making, terpolymers TPs derivedfrom three monomers including the two monomers constituting theaforesaid binary copolymers CPs may further be used in addition to thebinary copolymers CPs. Alternatively, such terpolymers TPs may be usedin place of the binary copolymers CPs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 includes schematic views illustrating a graded index type opticalfiber in accordance with the present invention. In FIG. 1, (a) is across-sectional view, (b) is a longitudinal sectional view, and (c) is adiagram showing the distribution of refractive indices in the radialdirection.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, HP represents a homopolymer, CP represents abinary copolymer, BP represents a mixture of two (co)polymers, L_(NB)represents a non-mixed layer formed of a single (co)polymer, and L_(B)represents a mixed layer formed of a mixture of two (co)polymers.

First of all, in order to facilitate the understanding of the presentinvention, a description is given of the embodiment in which the number(n) of monomers is 3. Where the number (n) of monomers is 3, threehomopolymers HP1, HP2, and HP3 are prepared from monomers M1, M2 and M3,respectively. Moreover, two series of binary copolymers CP1/2 and CP2/3are prepared from combinations of monomers giving homopolymers havingrefractive indices close to each other. It is preferable to select theseHPs and CPs so that each CP or HP has good compatibility with other CPs.

In the present invention, the polymers having higher refractive indicesare homopolymer HP1 derived from monomer M1, and binary copolymer CP1/2derived from monomers M1 and M2. With respect to CP1/2, a plurality ofcopolymers composed of the two monomers at different molar ratios andhaving different refractive indices may be prepared. Similarly, thepolymers having lower refractive indices are homopolymer HP3 derivedfrom monomer M3, and binary copolymer CP2/3 derived from monomers M2 andM3. Also with respect to CP2/3, a plurality of copolymers composed ofthe two monomers at different molar ratios and having differentrefractive indices may be prepared.

As illustrated in FIG. 1, the multilayer POFs of the present inventionhas a structure in which non-mixed layers (L_(NB)) having a thicknessT_(NB) and mixed layers (L_(B)) having a thickness T_(B) are alternatelyarranged. In this structure, each non-mixed layer (L_(NB)) is a layerformed of a single (co)polymer, and each mixed layer (L_(B)) is a layerformed of a mixture (BP) of the two (co)polymers constituting thenon-mixed layers disposed on both sides thereof.

If the number of non-mixed layers (L_(NB)) is increased, a structurehaving essentially no mixed layer (L_(B)) may be employed. However, whenthe number of non-mixed layers (L_(NB)) is small, it is necessary toform one or more mixed layers (L_(B)) and, moreover, increase theirthicknesses T_(B) to some degree so that an abrupt change in refractiveindex may be avoided.

FIG. 1 illustrates a POF having a five-layer structure comprising threenon-mixed layers (L_(NB)) and two mixed layers (L_(B)). As can be seenfrom FIG. 1(c), the refractive index remains constant in each non-mixedlayer (L_(NB)), while it changes continuously in each mixed layer(L_(B)). As the number of layers is increased, the refractive indexdistribution profile in the whole POF becomes more gradual. A gradualrefractive index distribution curve is preferable for the purpose ofincreasing the light transmission bandwidth. However, if the proportionof the mixed layers (L_(B)) in the POF is too large, its lighttransmission loss will be increased. Accordingly, the profile of therefractive index distribution is chosen with consideration for thebalance between the magnitude of the light transmission bandwidth andthe magnitude of the light transmission loss.

Moreover, a protective layer or a jacket material layer may be disposedon the outer periphery of the GI type POF, though they are not shown inFIG. 1.

First of all, BPs constituting the mixed layers (L_(B)) are explained.Generally, BPs tend to induce fluctuations in refractive index and aphase separation (which may hereinafter be suitably referred to as “aheterogeneous structure”), as compared with HPs and CPs. Consequently,the light scattering loss of the whole POF is increased as theproportion of LB in the POF becomes larger. Moreover, BPs generally hasworse thermal stability of than HPs and CPs. Consequently, when the POFis used in a relatively high temperature region for a long period oftime, the presence of L_(B) in the POF promotes the creation of aheterogeneous structure in the POF and increases its light scatteringloss.

Thus, since the light scattering loss of the whole POF is increased asthe proportion of L_(B) in the POF becomes larger, it is preferable thatthe proportion of L_(B) in the POF be smaller and the thickness T_(B) ofeach L_(B) be also smaller. The desirable value of T_(B) may varyaccording to the radial position of L_(B) and may also depend on thedesired bandwidth performance and the number of layers. However, T_(B)is preferably in the range of about 0.3 to 100 μm and more preferablyabout 1 to 10 μm.

It is also preferable that the HP (or CP) and CP forming each BP havegood compatibility and the difference in refractive index therebetweenbe sufficiently small.

Next, the polymers (i.e., HPs and CPs) constituting the non-mixed layers(L_(NB)) are explained. It is preferable that the (co)polymersconstituting L_(NB) in the POF have a small light scattering loss. Inorder to obtain (co)polymers having a small light scattering loss, thepolymers (or monomers) should preferably be chosen so that thedifference in refractive index between HP1 and HP2 and between HP3 andHP2 is as small as possible. The reason for this is that, if thedifference in refractive index between HP1 and HP2 (or between HP3 andHP2) is large, the polymer mixture (BP) of HP1 and HP2 or the copolymer(CP1/2) of M1 and M2 shows fluctuations in refractive index and hencecauses an increase in the light scattering loss of the POF.

Table 1 shows isotropic light scattering losses (dB/km) at a wavelengthof 650 nm for copolymers formed from 80 mole % of methyl methacrylate(MMA) used as M2 and 20 mole % of various monomers used as M1 or M3.Table 1 also shows the differences in refractive index (Δn_(d)) betweenthe homopolymers derived from these monomers and polymethyl methacrylate(PMMA). In this table, the Δn_(d) value is positive when the refractiveindex of the relevant homopolymer is larger than that of PMMA, andnegative when the refractive index of the relevant homopolymer issmaller than that of PMMA.

TABLE 1 Difference in refractive index Compositional Isotropic lightscat- (Δn_(d)) between Monomers ratio (wt. %) tering loss (dB/km)corresponding homopolymer MMA/VB 74.42/25.58 3725 0.0867 MMA/PhMA69.39/30.61 1867 0.0798 MMA/2-PhEMA 67.87/32.13 81.7 0.0684 MMA/BzA70.44/29.56 95.4 0.0676 MMA/GMA 74.04/25.96 10.2 0.0265 MMA/CEMA72.07/27.93 20.7 0.0262 MMA/THFMA 72.10/27.90 13.1 0.0188 MMA/CHMA72.60/27.40 13.5 0.0158 MMA 100 10.8 0.0000 MMA/IBMA 75.79/24.21 27.2−0.0138 MMA/TBMA 72.85/27.15 143.7 −0.0270

(Note 1)

VB: Vinyl benzoate

PhMA: Phenyl methacrylate

2-PhEMA: 2-Phenylethyl methacrylate

BzA: Benzyl acrylate

GMA: Glycidyl methacrylate

CEMA: Chloroethyl methacrylate

THFMA: Tetrahydrofurfuryl methacrylate

CHMA: Chlorohexyl methacrylate

IMBA: Isobutyl methacrylate

TBMA: tert-Butyl methacrylate

As is evident from this table, the isotropic light scattering losses ofthe copolymers tend to decrease as the absolute value of the differencein refractive index (Δn_(d)) becomes smaller. Accordingly, the twomonomers constituting each binary copolymer CP used in the POF of thepresent invention must be ones giving homopolymers HPs between whichthere is a small difference in refractive index. Specifically, thedifference in refractive index is preferably not greater than 0.03, morepreferably not greater than 0.02, and most preferably not greater than0.015. However, if the difference in refractive index is decreased to anundue extent, the NA will become too small. Accordingly, it is necessaryto select a combination of monomers M1 and M2 (or monomers M3 and M2)with consideration for this fact. For this reason, the difference inrefractive index is preferably not less than 0.010.

Moreover, in the multilayer POF of the present invention which includesmixed layers (L_(B)), an abrupt change in refractive index at the mixedlayers (L_(B)) is suppressed as the difference in refractive indexbetween adjacent non-mixed layers (L_(NB)) becomes smaller, and thisreduces the light scattering losses at the interfaces. Accordingly, itis preferable that the difference in refractive index between adjacentnon-mixed layers (L_(NB)) be as small as possible. Specifically, thedifference in refractive index is preferably not greater than 0.016 andmore preferably not greater than 0.008.

It is also preferable that BPs constituting the mixed layers (L_(B)) inthe POF have a small light scattering loss. A mixture having a smalllight scattering loss can be obtained by enhancing the mutualcompatibility of the (co)polymers being mixed.

One means to this end is to minimize the difference in copolymerizationratio between the HP (or CP) and CP constituting adjacent non-mixedlayers (L_(NB)). In a mixture BP composed of (co)polymers between whichthere is a large difference in copolymerization ratio, the properties ofone CP (or HP) are substantially different from those of the other CP.Consequently, their mutual compatibility is reduced and a heterogeneousstructure tends to be produced in the BP, resulting in an increasedlight scattering loss of the POF. Actually, the difference incopolymerization ratio is determined at a value which causessubstantially no problem for practical purposes, with consideration forthe proportion of the mixed layers (L_(B)) in the whole POF.

Table 2 shows isotropic light scattering losses at a wavelength of 650nm for BPs prepared by selecting two members from among HPs and variousCPs having different compositions and mixing them at a ratio of 50/50(wt. %). The aforesaid HPs and CPs were formed from 2,2,2-trifluoroethylmethacrylate (3FM) or 2,2,3,3-tetrafluoropropyl methacrylate (4FM) usedas M1, and 2,2,3,3,3-pentafluoropropyl methacrylate (5FM) used as M2.

In this table, the (co)polymer derived from monomers M1 and M2 is thehomopolymer HP1 of M1 when the content of M2 is 0 mole %, and thehomopolymer HP2 of M2 when the content of M1 is 0 mole %. The differencein copolymerization ratio between two copolymers A and B havingdifferent copolymerization ratios is expressed by the difference in themolar content (%) of M1 or M2.

Table 2 indicates that, as the copolymerization ratio of one CP (or HP)is closer to that of the other CP mixed therewith, the resulting BP hasa smaller isotropic light scattering loss. With respect to M1 or M2contained in any two adjacent (co)polymers, the difference incopolymerization ratio is preferably not greater than 20 mole %, morepreferably not greater than 15 mole %, and most preferably not greaterthan 10 mole %. However, if the difference in copolymerization ratio isextremely small, it may be necessary to increase the number of(co)polymer layers for the purpose of securing the desired NA of theoptical fiber.

TABLE 2 Monomer ratio Monomer ratio Difference in M1 con- Isotropiclight scattering Monomers of copolymer of copolymer tent betweencopolymers loss of mixture of co- M1/M2 1 (mole %) 2 (mole %) 1 and 2(mole %) polymers 1 and 2 (dB/km) 3FM/5FM 40/60 30/70 10 60-80  3FM/5FM45/55 30/70 15 70-100 3FM/5FM 50/50 30/70 20 80-140 3FM/5FM 50/50 0/10050 >10000 (cloudy) 3FM/5FM 50/50 100/0 50 >10000 (cloudy) 4FM/5FM 40/6030/70 10 60-80  4FM/5FM 45/55 30/70 15 80-110 4FM/5FM 50/50 30/70 2090-150 4FM/5FM 50/50 0/100 50 >10000 (cloudy) 4FM/5FM 50/50 100/050 >10000 (cloudy)

In the present invention, high or low refractive indices are used on arelative basis. For example, when MMA is used as M2 and, therefore, PMMAhaving a refractive index of 1.491 is used as HP2, the monomers whichcan be used as M1 and M3 are exemplified below. The n_(d) values givenin parentheses represent the refractive indices of the correspondinghomopolymers.

Examples of monomer M1 used to form a polymer having a high refractiveindex include benzyl methacrylate (n_(d)=1.5680), phenyl methacrylate(n_(d)=1.5706), vinyl benzoate (_(nd)=1.5775), styrene (n_(d)=1.5920),1-phenylethyl methacrylate (n_(d)=1.5490), 2-phenylethyl methacrylate(n_(d)=1.5592), diphenylmethyl methacrylate (n_(d)=1.5933),1,2-diphenylethyl methacrylate (n_(d)=1.5816), 1-bromoethyl methacrylate(n_(d)=1.5426), benzyl acrylate (n_(d)=1.5584), α, α-dimethylbenzylmethacrylate (n_(d)=1.5820), p-fluorostyrene (n_(d)=1.566),2-chloroethyl methacrylate (n_(d)=1.5170), isobornyl methacrylate(n_(d)=1.505), adamantyl methacrylate (n_(d)=1.535), tricylodecylmethacrylate (n_(d)=1.523), 1-methylcyclohexyl methacrylate(n_(d)=1.5111), 2-chlorocyclohexyl methacrylate (n_(d)=1.5179),1,3-dichloropropyl methacrylate (n_(d)=1.5270),2-chloro-1-chloromethylethyl methacrylate (n_(d)=1.5270), bornylmethacrylate (n_(d)=1.5059), cyclohexyl methacrylate (n_(d)=1.5066),tetrahydrofurfyl methacrylate (n_(d)=1.5096), allyl methacrylate(n_(d)=1.5196), tetrahydrofurfuryl methacrylate (n_(d)=1.5096), vinylchloroacetate (n_(d)=1.5120), glycidyl methacrylate (n_(d)=1.517) andmethyl α-chloroacrylate (n_(d)=1.5172).

Examples of monomer M3 used to form a polymer having a low refractiveindex include 2,2,2-trifluoroethyl methacrylate (n_(d)=1.415),2,2,3,3-tetrafluoropropyl methacrylate (n_(d)=1.422),2,2,3,3,3-pentafluoropropyl methacrylate (n_(d)=1.392),2,2,2-trifluoro-1-trifluoromethylethyl methacrylate (n_(d)=1.380),2,2,3,4,4,4-hexafluorobutyl methacrylate (n_(d)=1.407),2,2,3,3,4,4,5,5-octafluoropentyl methacrylate (n_(d)=1.393),2,2,2-trifluoroethyl α-fluoroacrylate (n_(d)=1.386),2,2,3,3-tetrafluoropropyl α-fluoroacrylate (n_(d)=1.397),2,2,3,3,3-pentafluoropropyl α-fluoroacrylate (n_(d)=1.366),2,2,3,3,4,4,5,5-octafluoropentyl α-fluoroacrylate (n_(d)=1.376), o- orp-difluorostyrene (n_(d)=1.4750), vinyl acetate (n_(d)=1.4665),tert-butyl methacrylate (n_(d)=1.4638), isopropyl methacrylate(n_(d)=1.4728), hexadecyl methacrylate (n_(d)=1.4750), isobutylmethacrylate (n_(d)=1.4770), α-trifluoromethylacrylates,β-fluoroacrylates, β,β-difluoroacrylates, β-trifluoromethylacrylates,β,β-bis(trifluoromethyl)acrylates and α-chloroacrylates.

Preferably, the monomers used to prepare the (co)polymers constitutingthe GI type POF of the present invention are ones giving homopolymerswith a glass transition temperature (Tg) of 70° C. or above. If Tg isunduly low, the thermal resistance of the whole POF will be reduced. Asa result, there is a possibility that, in a service environment havingrelatively high temperatures, phase separation, especially in the L_(B)layers, may be accelerated to cause an increase in scattering loss.Examples of such high-Tg (co)polymers include (co)polymers derived froma combination of methyl methacrylate and chloroethyl methacrylate.

Especially preferred examples of (co)polymers which have a smalldifference in refractive index between HPs and hence cause a smallscattering loss in POFs include (co)polymers derived from a combinationof two or three fluoroalkyl (meth)acrylates. Similarly, they alsoinclude (co)polymers derived from a combination of monomers selectedfrom chlorohexyl methacrylate, tetrahydrofurfuryl methacrylate, glycidylmethacrylate, isobutyl methacrylate and methyl methacrylate, and havingdifferent copolymerization ratios.

Furthermore, examples of (co)polymers which have a large difference inrefractive index between HPs but exhibit good compatibility include(co)polymers derived from 2-phenylethyl methacrylate and methylmethacrylate, and having different copolymerization ratios.

No particular limitation is placed on the difference in refractive indexbetween the center and the outer periphery of the GI type POF of thepresent invention. However, in view of the magnitude of the numericalaperture (NA), it is preferable that the difference in refractive indexbe in the range of about 0.02 to 0.04.

Now, the method of making a GI type POF in accordance with the presentinvention is described below.

According to this method, each spining material is prepared from one(co)polymer and three or more, preferably five or more, spinningmaterials having different refractive indices are prepared by using(co)polymers selected from the group consisting of two or morehomopolymers HP1, HP2, . . . , HPn (in which n is an integer of 2 orgreater) derived from monomers M1, M2, . . . , Mn, respectively, andhaving refractive indices decreasing in that order, and one or morebinary copolymers CPs derived from the monomers. Then, these spinningmaterials are fed to a multilayer concentric circular nozzle havingthree or more, preferably five or more, layers so that the refractiveindex decreases toward the outer periphery, and thereby extruded throughthe nozzle.

In order to create a gradual refractive index distribution profilebetween adjacent layers, mixed layers must be formed bypolymer-to-polymer interdiffusion between adjacent layers. To this end,the following procedure is employed. For example, the spinning materialsare melted within the spinning nozzle, and the spinning materialsconstituting any two adjacent layers are brought into contact with eachother for a relatively long period of time to effect polymer-to-polymerinterdiffusion, and then extruded therefrom. However, when the number oflayers is sufficiently large, no positive treatment for effectingpolymer-to-polymer interdif fusion between adjacent layers is required.

Where a gradual refractive index distribution curve is not obtainedowing to insufficient interdiffusion within the nozzle, the extrudedfiber may be heat-treated again to effect additional polymer-to-polymerinterdiffusion. However, when this method is employed, the fiber shouldpreferably be extruded from the spinning nozzle in an undrawn state soas to prevent relaxation shrinkage of the fiber during heat treatment.The reason for this is that change in fiber diameter increase the lighttransmission loss of the POF.

The heat treatment may be carried out, for example, in the followingmanner. First, the undrawn fiber is heat-treated at a temperature over100° C. higher than the average glass transition temperature (Tg) of the(co)polymers constituting it to effect interdiffusion. Then, the fiberis drawn in a temperature range extending from Tg to a temperature about80° C. higher than Tg, so as to impart flexural strength to the fiber.Thus, there can be obtained a GI type POF.

Furthermore, in order to increase the thicknesses of the mixed layers,there may be employed a method which comprises adding to each spinningmaterial a monomer mixture having the same composition as the(co)polymer constituting the spinning material and a photopolymerizationinitiator, extruding the resulting spinning materials through a nozzleso as to allow the monomers to interdiffuse between adjacent layers, andthen photopolymerizing the monomers within the fiber.

The refractive index profile of the POF can be controlled by varying theresidence time within the spinning nozzle, the melt spinningtemperature, the post-spinning heat-treating temperature, the draw ratioduring spinning, the types of the resinous components, and the number ofconcentric cylindrical layers of spinning materials (hereinafterreferred to as “spinning material layers”).

Now, the design method for manufacturing a GI type POF having an idealrefractive index profile (i.e., the conditions giving the widestbandwidth) is described below with respect to the relationship betweenthe multilayer concentric cylindrical arrangement of spinning materialswithin the spinning nozzle and the refractive indices thereof. However,it is to be understood that the present invention is not limited by thefollowing description.

Let us consider a GI type POF in which the refractive index decreasesgradually from the center toward the outer periphery. If the refractiveindex at the center is designated by n₁, the lowest refractive index atthe outer periphery by n₂, the radius by (a), and the position (ordistance) from the center by r (0<r<a), and if it is assumed thatΔ=(n₁−n₂)/n₁ the conditions which impart the widest bandwidth to the POFare such that the refractive index profile, n(r), is approximated by thefollowing equation.

n(r)=n ₁{1−2Δ(r/a)²}^(0.5)  (1)

That is, if the values of n₁, n₂ and (a) are determined, the idealrefractive index profile within the POF can be determined according toequation (1). Moreover, if the ratio of the diameter (b) of the spinningnozzle to the diameter (c) of the extruded and drawn POF is designatedby α(1<α=b/c), the refractive index profile, n′(r), to be formed withinthe spinning nozzle (in which the core diameter is αa) is described bythe following equation.

n′(r)=n ₁{1−2Δ(r/αa)²}^(0.5)  (2)

Accordingly, the radial position r_(j) (j=1, 2, 3, . . . ) in thespinning nozzle at which a spinning material polymer j having arefractive index n′_(j) is arranged can be determined by substitutingn′_(j) for n′(r) and r_(j) for r in equation (2). Thus, the followingequation is obtained.

r _(j) =αa[{1−(n′ _(j) /n ₁)²}/2Δ]^(0.5)  (3)

In this case, the number (N) of spinning material layers depends on thecore radius (αa) within the nozzle and the interdiffusion distance (L)of the spinning material polymers. It is reasonable that N is equal to(αa/2L). If (αa) is significantly large as compared with L, this wouldbe rather undesirable because feeder of the spinning material polymersto the nozzle and control of the spinning conditions are complicated tocause an increase in production cost. Moreover, if N <<αa/2L, theinterdiffusion distance will be short relative to the thicknesses of thespinning material layers. Consequently, the desired refractive indexprofile cannot be satisfactorily formed, so that the resulting POF willhave a worse transmission bandwidth. However, to avoid a high productioncost and a troublesome production process, multilayer spinningcomprising about 5 to 10 layers is considered to be proper from apractical point of view. The POF formed in this manner has a somewhatstepwise refractive index profile. Its bandwidth performance does notreach that of a POF having the ideal refractive index profile ofequation (1), but fully meets the requirements for practical purposes.

According to the method of the present invention, a multicore fiber mayalso be formed by extruding such multilayer fibers simultaneouslythrough a plurality of nozzles disposed in close proximity to eachother.

While the embodiment in which the number (n) of monomers is 3 has beendescribed above, the difference in refractive index between the centerand the outer periphery of a GI type POF can be easily increased byincreasing n to 4 or greater, so that a higher NA can be achievedeasily.

Moreover, even if the number (n) of monomers is 2, a GI type POF havinga small light scattering loss can be formed by selecting a combinationof two monomers giving homopolymers between which there is a smalldifference in refractive index.

As the (co)polymers constituting the non-mixed layers (L_(NB)) of the GItype POF of the present invention, terpolymers TPs may also be used inorder, for example, to improve the thermal resistance and mechanicalstrength of the POF. That is, terpolymers TPs derived from threemonomers including the two monomers constituting the aforesaid binarycopolymers CPs may further be used in addition to the binary copolymersCPs. Alternatively, such terpolymers TPs may be used in place of thebinary copolymers CPs.

The present invention is further illustrated by the following examples.

EXAMPLE 1

Four monomeric components were used in this example. They includedglycidyl methacrylate (GMA) giving a homopolymer with a refractive index(n_(d)) of 1.5174 and a glass transition temperature (Tg) of 46° C.,cyclohexyl methacrylate (CHMA) giving a homopolymer with an n_(d) of1.5066 and a Tg of 83° C., MMA giving a homopolymer with an n_(d) of1.4908 and a Tg of 112° C., and isobutyl methacrylate (IBMA) giving ahomopolymer with an n_(d) of 1.4770 and a Tg of 48-53° C. In each binarycopolymers, therefore, the difference in refractive index (Δn_(d))between the two homopolymers was as follows.

GMA/CHM (Δn_(d)=0.0108)

CHMA/MMA (Δn_(d)=0.0158)

MMA/IBMA (Δn_(d)=0.0138)

The following eight monomers and monomer mixtures (with mixing ratiosexpressed in percent by weight) were subjected to polymerizationreaction.

1) GMA/CHMA=17.44/82.56

2) CHMA

3) CHMA/MMA=87.05/12.95

4) CHMA/MMA=71.59/28.41

5) CHMA/MMA=52.83/47.17

6) CHMA/MMA=29.58/70.42

7) MMA

8) MMA/IBMA=73.80/26.20

Monomer mixture solutions were prepared by adding 500 μl of n-dodecylmercaptan as a molecular weight controller (or chain transfer agent) to100 g of each of the monomers or monomer mixtures, and further addingthereto 0.11 g of azobis(dimethylvaleronitrile) as a low-temperatureinitiator and 8.00 μl of di-tert-butyl peroxide as a high-temperatureinitiator. In order to obtain polymers useful as spinning materials,these monomer mixture solutions were subjected to two-step radicalpolymerization. That is, they were polymerized under an atmosphere ofnitrogen at 70° C. for 5 hours in such a way as to cause no foaming.After the degree of polymerization reached 90% by weight or greater,they were polymerized at 130° C. for 40 hours. The resulting polymershad a weight-average molecular weight of about 100,000 to 140,000 on thebasis of measurements by GPC, and their residual monomer content was 1%by weight or less.

Subsequently, these eight spinning materials were fed to an extruder,melted at 240° C., and extruded through a composite spinning nozzlehaving an eight-layer concentric cylindrical structure. This spinningnozzle is designed so that an eight-layer concentric cylindricalstructure is formed at a position 500 mm before the nozzle tip fromwhich the fiber in its molten state is extruded. Moreover, this nozzleis fabricated so that its internal diameter decreases gradually over alength of 100 mm extending from the aforesaid position in the directionof extrusion. Finally, starting from a position 400 mm before the tip,the diameter of the nozzle remains constant at 2 mm. Basically, agradual refractive index distribution profile is created bypolymer-to-polymer interdiffusion while the molten polymers flow throughthis 400 mm section. The temperature of this spinning nozzle section isstrictly controlled by dividing it into four equal subsections having alength of 100 mm. The temperature of the 100 mm subsection adjoining thespinning nozzle tip was adjusted to 230° C. so as to secure thestability of spinning, and the temperature of the other threesubsections was adjusted to 240° C. so as to promote thepolymer-to-polymer interdiffusion.

The extrusion speed of the polymers was 40 mm/min and the residence timeof the polymers in the spinning nozzle section having a diameter of 2 mmwas about 10 minutes. The extruded fiber was drawn so as to give a finaldiameter of 1 mm, and taken up by means of a wind-up machine.

The POF formed in the above-described manner was cut at a length of 0.1km to measure its −3 dB transmission bandwidth. Thus, it was found to be900 MHz. This transmission bandwidth measurement was made at a launch NAof 0.85 by using an optical sampling oscilloscope (manufactured byHamamatsu Photonics Co., Ltd.) and a light. source comprising aSemiconductor Laser TOLD 9410 (manufactured by Toshiba Corp.) with anemission wavelength of 650 nm. Moreover, its transmission loss was 160dB/km. This transmission loss measurement was made at a wavelength of650 nm and a launch NA of 0.1 according to the 100 m/5 m cut-backmethod. The same measuring conditions were also employed in thefollowing examples.

The numerical aperture (NA) of this GI type POF was 0.25. Moreover, thethickness of each mixed layer in the POF was about 1-3 μm.

EXAMPLE 2

A multicore fiber having a sea-and-island structure was made by using,as the islands, nine POFs each of which has the same multilayerstructure as described in Example 1. However, the copolymer composed ofMMA and IBMA in a ratio of 73.80:26.20 and disposed on the outermostside in Example 1 was used as the sea material. Accordingly, except forthe sea material, the structure of the islands consisted essentially ofthe part of the fiber of Example 1 extending from its center to theseventh layer. The average diameter of the islands was about 0.5 mm, andthe diameter of the whole multicore fiber was 3.0 mm. The transmissionloss of this multicore fiber was 250 dB/km, and its transmissionbandwidth per island at a length of 0.1 km was 650 MHz. The thickness ofeach mixed layer in the POFs was about 1-3 μm.

EXAMPLE 3

Three monomeric components were used in this example. They included2,2,3,3-tetrafluoropropyl methacrylate (4FM) giving a homopolymer with arefractive index (n_(d)) of 1.4215 and a Tg of 64° C.,2,2,3,3,3-pentafluoropropyl methacrylate (5FM) giving a homopolymer withan n_(d) of 1.3920 and a Tg of 67° C., and 2-(perfluorooctyl)ethylmethacrylate (17FM) giving a homopolymer with an n_(d) of 1.3732. Ineach binary copolymer system, therefore, the difference in refractiveindex (Δn_(d)) between the two homopolymers was as follows.

4FM/5FM (Δn_(d)=0.0295)

5FM/17FM (Δn_(d)=0.0188)

The following eight monomer and monomer mixtures (with mixing ratiosexpressed in percent by weight) were subjected to polymerizationreaction.

1) 4FM/5FM=57.92/42.08

2) 4FM/5FM=45.86/54.14

3) 4FM/5FM=34.04/65.96

4) 4FM/5FM=22.46/77.54

5) 4FM/5FM=11.12/88.88

6) 5FM

7) 5FM/17FM=78.67/21.33

8) 5FM/17FM=62.11/37.89

According to the same procedure as described in Example 1, thesemonomers and monomer mixtures were polymerized and spun to form a POF.The transmission bandwidth of this POF was 1.1 GHz, its transmissionloss was 140 dB/km, and the thickness of each mixed layer was about 1-3μm.

EXAMPLE 4

Two monomeric components were used in this example. They included2,2,2-trifluoroethyl methacrylate (3FM) giving a homopolymer with arefractive index (n_(d)) of 1.4146 and a Tg of 75° C., and2,2,3,3,3-pentafluoropropyl methacrylate (5FM) giving a homopolymer withan n_(d) of 1.3920 and a Tg of 67° C. In the binary copolymer,therefore, the difference in refractive index (Δn_(d)) between the twohomopolymers was 0.0226. The following eight monomers and monomermixtures (with mixing ratios expressed in percent by weight) weresubjected to polymerization reaction.

1) 3FM

2) 3FM/5FM=82.56/17.44

3) 3FM/5FM=66.46/33.54

4) 3FM/5FM=51.56/48.44

5) 3FM/5FM=37.72/62.28

6) 3FM/5FM=24.83/75.17

7) 3FM/5FM=12.80/87.20

8) 5FM

According to the same procedure as described in Example 1, thesemonomers and monomer mixtures were polymerized and spun to form a POF.The transmission bandwidth of this POF was 1.9 GHz, its transmissionloss was 110 dB/km, and the thickness of each mixed layer was about 1-3μm.

EXAMPLE 5

Two monomers, i.e. 4FM and 5FM, were used in this example. The followingeight monomer and monomer mixtures (with mixing ratios expressed in molepercent) were subjected to polymerization reaction. In this case, thedifference in refractive index (Δn_(d)) between the two homopolymers was0.0295.

1) 4FM/5FM=70/30

2) 4FM/5FM=60/40

3) 4FM/5FM=50/50

4) 4FM/5FM=40/60

5) 4FM/5FM=30/70

6) 5FM/5FM=20/80

7) 5FM/5FM=10/90

8) 5FM

Using the resulting eight polymers as spinning materials, a POF wasformed in the same manner as described in Example 1. The transmissionbandwidth of this POF was 1.5 GHz, its transmission loss was 120 dB/km,and the thickness of each mixed layer was about 1-3 μm.

EXAMPLE 6

Two monomeric components were used in this example. They includedchloroethyl methacrylate (CEMA) giving a homopolymer with an n_(d) of1.517 and a Tg of 92° C., and MMA giving a homopolymer with an n_(d) of1.491 and a Tg of 112° C. The following eight monomer and monomermixtures (with mixing ratios expressed in mole percent) were subjectedto polymerization reaction. In this case, the difference in refractiveindex (Δn_(d)) between the two homopolymers was 0.026.

1) CEMA/MMA=84/16

2) CEMA/MMA=72/28

3) CEMA/MMA=60/40

4) CEMA/MMA=48/52

5) CEMA/MMA=36/64

6) CEMA/MMA=24/76

7) CEMA/MMA=12/89

8) MMA

Using the resulting eight polymers as spinning materials, a POF wasformed in the same manner as described in Example 1. The transmissionbandwidth of this POF was 1.2 GHz, its transmission loss was 155 dB/km,and the thickness of each mixed layer was about 1-3 μm.

EXAMPLE 7

Three monomeric components were used in this example. They includedtetrahydrofurfuryl methacrylate (THFMA) giving a homopolymer with ann_(d) of 1.510 and a Tg of 60° C., MMA giving a homopolymer with ann_(d) of 1.491 and a Tg of 112° C., and isobutyl methacrylate (IBMA)giving a homopolymer with an n_(d) of 1.477 and a Tg of 48-53° C. Thefollowing eight monomer and monomer mixtures (with mixing ratiosexpressed in mole percent) were subjected to polymerization reaction.

1) THFMA/MMA=80/20

2) THFMA/MMA=60/40

3) THFMA/MMA=40/60

4) THFMA/MMA=20/80

5) MMA

6) MMA/IBMA=80/20

7) MMA/IBMA=60/40

8) MMA/IBMA=40/60

Using the resulting eight polymers as spinning materials, a POF wasformed by spinning them in the same manner as described in Example 1.The transmission bandwidth of this POF was 1.2 GHz, its transmissionloss was 190 dB/km, and the thickness of each mixed layer was about 1-3μm.

EXAMPLE 8

Two monomeric components were used in this example. They included2-phenylethyl methacrylate (2-PhEMA) giving a homopolymer with an n_(d)of 1.559, and MMA giving a homopolymer with an n_(d) of 1.491 and a Tgof 112° C. The following eight monomer and monomer mixtures (with mixingratios expressed in mole percent) were subjected to polymerizationreaction.

1) 2-PhEMA/MMA=35/65

2) 2-PhEMA/MMA=30/70

3) 2-PhEMA/MMA=25/75

4) 2-PhEMA/MMA=20/80

5) 2-PhEMA/MMA=15/85

6) 2-PhEMA/MMA=10/90

7) 2-PhEMA/MMA=5/95

8) MMA

Using the resulting eight polymers as spinning materials, a POF wasformed by spinning them in the same manner as described in Example 1.The transmission bandwidth of this POF was 1.3 GHz, its transmissionloss was 200 dB/km, and the thickness of each mixed layer was about 1-3μm.

EXAMPLE 9

Two monomeric components were used in this example. They included2,2,2-trifluoro-1-trifluoromethylethyl methacrylate (iso-6FM) giving ahomopolymer with an n_(d) of 1.380 and a Tg of 78° C., and2,2,2-trifluoethyl methacrylate (3FM) giving a homopolymer with an n_(d)of 1.415 and a Tg of 75° C. The following eight monomer and monomermixtures (with mixing ratios expressed in mole percent) were subjectedto polymerization reaction.

1) 3FM

2) iso-6FM/3FM=10/90

3) iso-6FM/3FM=20/80

4) iso-6FM/3FM=30/70

5) iso-6FM/3FM=40/60

6) iso-6FM/3FM=50/50

7) iso-6FM/3FM=60/40

8) iso-6FM/3FM=70/30

Using the resulting eight polymers as spinning materials, a POF wasformed by spinning them in the same manner as described in Example 1.The transmission bandwidth of this POF was 1.0 GHz, its transmissionloss was 130 dB/km, and the thickness of each mixed layer was about 1-3μm.

EXAMPLE 10

Two monomeric components were used in this example. They includedchloroethyl methacrylate (CEMA) giving a homopolymer with an n_(d) of1.517 and a Tg of 92° C., and methyl methacrylate (MMA) giving ahomopolymer with an n_(d) of 1.491 and a Tg of 112° C. The following sixmonomer and monomer mixtures (with mixing ratios expressed in molepercent) were subjected to polymerization reaction.

1) CEMA/MMA=80/20

2) CEMA/MMA=64/36

3) CEMA/MMA=48/52

4) CEMA/MMA=32/68

5) CEMA/MMA=16/84

6) MMA

Each of these six monomer and monomer mixtures was thermally polymerizeduntil a degree of polymerization of about 50% was reached. Thus, highlyviscous monomer/polymer mixed syrups were prepared.

Subsequently, after the addition of a photopolymerization initiator,these six mixed syrups were fed to the same multilayer spinning nozzleas used in Example 1, except that the spinning nozzle had a six-layerconcentric cylindrical structure and its temperature was adjusted to 40°C. After being extruded, the aforesaid syrups were photopolymerized byUV irradiation. Thus, their polymerization was completed to form a POF.

The transmission bandwidth of this POF was 2.1 GHz, its transmissionloss was 140 dB/km, and the thickness of each mixed layer was about 30μm.

EXAMPLE 11

Three monomeric components were used in this example. They includedcyclohexyl methacrylate (CHMA) giving a homopolymer with an n_(d) of1.5066 and a Tg of 83° C., MMA giving a homopolymer with an n_(d) of1.491 and a Tg of 112° C., and isobutyl methacrylate (IBMA) giving ahomopolymer with an n_(d) of 1.477 and a Tg of 48-53° C. The followingeight monomers and monomer mixtures (with mixing ratios expressed inmole percent) were subjected to polymerization reaction.

1) CHMA/IBMA/MMA=70/10/20

2) CHMA/IBMA/MMA=60/20/20

3) CHMA/IBMA/MMA=50/30/20

4) CHMA/IBMA/MMA=40/40/20

5) CHMA/IBMA/MMA=30/50/20

6) CHMA/IBMA/MMA=20/60/20

7) CHMA/IBMA/MMA=10/70/20

8) CHMA/IBMA/MMA=0/80/20

Using the resulting eight polymers as spinning materials, a POF wasformed by spinning them in the same manner as described in Example 1.The transmission bandwidth of this POF was 1.1 GHZ, its transmissionloss was 180 dB/km, and the thickness of each mixed layer was about 1-3μm.

Exploitability in Industry

The present invention can provide GI type POFs having a small lightscattering loss and a relatively large numerical aperture. Moreover, themethod for forming POFs in accordance with the present invention hashigh productivity.

What is claimed is:
 1. A graded index type optical fiber having amultilayer structure, comprising: a plurality of concentrically arrangedlayers, each of said layers being formed of one (co)polymer selectedfrom the group consisting of two or more (homo)polymers HP1, HP2, . . .Hpn, wherein n is an integer of 2 or greater, derived from monomers M1,M2, . . . , Mn, respectively, and having refractive indices decreasingin that order, and one or more copolymers derived from said monomers,said multilayer structure being such that a mixed layer consisting of amixture of two (co)polymers each of which constitutes the polymer of oneof the two adjacent layers is formed therebetween, and the refractiveindex is highest at the center and decreases gradually toward the outerperiphery.
 2. An optical fiber as claimed in claim 1, wherein saidcopolymers and homopolymers are derived from monomers each giving ahomopolymer with a glass transition temperature of 70° C. or above. 3.An optical fiber as claimed in claim 1, wherein said optical fiber has amultilayer structure comprising three or more concentrically arrangedlayers formed of (co)polymers which are derived from two monomers givinghomopolymers with a difference in refractive index of 0.03 or less, andwhich have different copolymerization ratios and refractive indices. 4.An optical fiber as claimed in claim 1, wherein the difference inrefractive index between the (co)polymers constituting any adjacentlayer is 0.016 or less.
 5. An optical fiber as claimed in claim 1, whichis formed of (co)polymers derived from a combination of monomersselected from the group consisting of chlorohexyl methacrylate,tetrahydrofurfuryl methacrylate, glycidyl methacrylate, isobutylmethacrylate and methyl methacrylate.
 6. An optical fiber as claimed inclaim 1, wherein the difference in copolymerization ratio between the(co)polymers constituting any adjacent layers is not greater than 20mole %.
 7. An optical fiber as claimed in claim 1, wherein at least twoof the monomers M1, M2, . . . , Mn are fluoroalkyl (meth)acrylates.
 8. Amulticore optical fiber having a sea-and-island structure comprising anisland component consisting of the graded index type optical fiber asclaimed in claim 1 and a sea component in which a plurality of theisland components are disposed.
 9. A method of making a graded indextype optical fiber, which comprises: preparing a plurality of spinningmaterials having different refractive indices, each of said spinningmaterials being made of one (co)polymer, by using (co)polymers selectedfrom the group consisting of two or more (homo)polymers HP1, HP2, . . .Hpn, wherein n is an integer of 2 or greater, derived from monomers M1,M2, . . . , Mn, respectively, and having refractive indices decreasingin that order, and one or more copolymers derived from said monomers;feeding said spinning materials to a multilayer concentric circularnozzle so that the refractive index decreases toward the outerperiphery, and thereby extruding them through the nozzle; and allowingthe polymers to interdiffuse between adjacent layers of the fiber withinsaid nozzle, after extrusion from the nozzle or within the nozzle andafter the polymer material is extruded from the nozzle.
 10. A method ofmaking an optical fiber as claimed in claim 9, wherein five or more(co)polymers having different refractive indices are used.
 11. A methodof making an optical fiber as claimed in claim 9, wherein three or more(co)polymers having different copolymerization ratios and refractiveindices are used, said (co)polymers being derived from two monomersgiving homopolymers HP1 and HP2 in which the difference in refractiveindex is not greater than 0.03.
 12. A method of making an optical fiberas claimed in claim 9, wherein the (co)polymers are derived from two ormore monomers giving homopolymers in which the difference in refractiveindex between two homopolymers having the refractive indices closest toeach other is not greater than 0.02.
 13. A method of making an opticalfiber as claimed in claim 9, wherein the difference in refractive indexbetween two (co)polymers fed to adjacent nozzle orifices of saidmultilayer concentric circular nozzle is not greater than 0.016.
 14. Amethod of making an optical fiber as claimed in claim 9, wherein thedifference in copolymerization ratio between two (co)polymers fed toadjacent nozzle orifices of said multilayer concentric circular nozzleis not greater than 20 mole %.
 15. A method of making a graded indextype optical fiber, which comprises: preparing a plurality of spinningmaterials having different refractive indices, each of said spinningmaterials being made of one (co)polymer, each of said spinning materialscontaining a (co)polymer selected from the group consisting of two ormore (homo)polymers HP1, HP2, . . . Hpn, wherein n is an integer of 2 orgreater, derived from monomers M1, M2, . . . , Mn, respectively, andhaving refractive indices decreasing in that order, and one or morecopolymers derived from said monomers, and further containing monomermixtures having the same composition as each of said (co)polymer, and aphotopolymerization initiator; feeding said spinning materials to amultilayer concentric circular nozzle so that the refractive indexdecreases toward the outer periphery, and thereby extruding them throughthe nozzle; and allowing said monomers to interdiffuse between adjacentlayers of the fiber; and photopolymerizing said monomers.
 16. A methodof making a graded index type optical fiber, which comprises: preparinga plurality of spinning materials having different refractive indices,each of said spinning materials being made of one (co)polymer, by using(co)polymers selected from the group consisting of three or more(homo)polymers HP1, HP2, . . . Hpn, wherein n is an integer of 3 orgreater, derived from monomers M1, M2, . . . , Mn, respectively, andhaving refractive indices decreasing in that order, and one or morecopolymers derived from said monomers; feeding said spinning materialsto a multilayer concentric circular nozzle so that the refractive indexdecreases toward the outer periphery, and thereby extruding them throughsaid nozzle; and allowing the polymers to interdiffuse between adjacentlayers of the fiber within said nozzle, after extrusion from the nozzleor within the nozzle and after the polymer material is extruded from thenozzle.